The title compound, [Al(C5H8N2)Cl3], was prepared by a thermolytic decomposition under high-vacuum conditions and presents a formal adduct of an Arduengo carbene, 1,3-dimethyl-1H-imidazol-2-yl­idene, and aluminium trichloride. The Al atom adopts a pseudo-tetra­hedral CCl3 coordination environment. All N and C atoms, the Al atom, one of the Cl atoms, and all aromatic H atoms of the mol­ecule lie on a mirror plane. As a result of the mirror symmetry of the mol­ecule, the H atoms of all methyl groups are disordered between symmetry-equivalent positions.

Structurally characterized AlX3 (X = Cl, I) adducts with Arduengo carbenes (ACs) are known since 2004 (Stasch et al., 2004) and are still few (Ghadwal et al., 2009; Bantu et al., 2009). Except of the first described representative, viz. trichlorido(1,3,4,5-dimethyl-2,3-dihydro-1H-imidazol-2-ylidene-κC2)aluminium (Stasch et al., 2004), they all present complexes with sterically hindered ACs which bear either mesityl or 2,6-diisopropylphenyl substituents at the N-atoms. Of interest, in all four structurally characterized X3Al—AC adducts, the Al atoms are in a tetrahedral coordination environment as shown by an analysis of the structures compiled in the Cambridge Structural Database (Allen, 2002) [CSD; Version 5.34, release May 2013; 4 entries, 4 fragments].

Preparation of all Al complexes mentioned above includes, as a step, generation of a free AC by deprotonation of a corresponding imidazolium salt with a strong base under mild conditions. This seriously limits the method due to the known thermal instability of sterically non-hindered ACs even in solution. Recently, we developed a facile route to BF3 and PF5 adducts with sterically non-hindered ACs by a thermolytic decomposition of related imidazolium salts with [BF4-] and/or [PF5-] anions under high-vacuum conditions [573–673 K, 1.3–2.0×10 -1 Pa; Tian et al. (2012)]. Heating of an equimolar mixture of 1,3-dimethyl-1H-imidazolium (hydrogen difluoride), [C5H8N2+][HF2-], (II), and AlCl3 under the same conditions followed by re-crystallization precedures led to formation of the title compound (I), C5H8N2AlCl3, in a moderate yield (see Experimental for the details; for the crystal structure of (II), see: Tian et al., 2013).

Compound (I) presents a formal adduct of an Arduengo carbene, 1,3-dimethyl-1H-imidazol-2-ylidene, and aluminium trichloride. The Al-atom adopts a pseudo-tetrahedral coordination environment, defined by the three Cl atoms and the carbene C atom (Table 1). All N- and C-atoms, the Al-atom, one of the Cl-atoms, and all aromatic H-atoms of the molecule lie on a mirror plane at (x, 1/4, z). The H-atoms of methyl groups are disordered between symmetry equivalent positions (Fig. 1).

Acetonitrile and toluene solvents were kept over and distilled from CaH2 and/or Na/K alloy under Ar atmosphere, respectively. AlCl3 was sublimed in high vacuum prior to use. 1H NMR spectra were recorded on a Varian 400 INOVA instrument in CD3CN at 400 MHz and 298 K, with the signal of the residual solvent protons [δ(H) = 1.94 p.p.m.] used as an internal reference.

Compound (I): Crystalline (II) (0.93 g, 6.8 mmol) and AlCl3 (0.91 g, 6.8 mmol) were placed into a small apparatus for distillation of high-melting compounds, the system was connected to the high-vacuum line (1.3–2.0×10 -1 Pa) through a liq. N2 cooled trap, evacuated, and heated. At approximately 423 K, the signs of melting were observed and then a strongly exothermic reaction accompanied by a gas evolution started. The temperature of the reaction mixture was then increased up to 573 K and the crude product was collected in the receiver as a reddish oil (reaction vessel temperature 573–673 K). The crude compound of (I) (1.03 g, 4.5 mmol, 66%) was allowed to crystallize in a refrigerator (270 K, 7 days). 1H NMR δ p.p.m.: 3.98 (s, 6H, CH3), 7.24 (s, 2H, CH=CH). Single crystal of (I) suitable for the X-ray diffraction analysis were grown by recrystallization of crude (I) from dry toluene, mounted in a Lindemann glass capillary (Ø 0.5 mm, glove box, N2 atmosphere) and sealed off.

The H atoms were treated as riding atoms with distances C—H = 0.96 (CH3), 0.93 Å (CArH) and Uiso(H) = 1.5 Ueq(C), 1.2 Ueq(C), respectively. The H-atoms of the methyl groups are disordered due to the mirror symmetry of the aromatic ring; hance they were refined with half-occupancy.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Footnotes

‡Previous address: Key Laboratory of Synthetic and Natural Chemistry of the Ministry of Education, College of Chemistry and Material Science, the North-West University of Xi'an, Taibai Bei Avenue 229, Xi'an 710069, Shaanxi Province, People's Republic of China.

Acknowledgements

Financial support from the National Natural Science Foundation of China (project Nos. 20702041 and 21072157) and the Shaanxi Province Administration of Foreign Experts Bureau Foundation (grant No. 20106100079) is gratefully acknowledged. The authors are thankful to Mr Pengfei Su (Xi'an Modern Chemistry Research Institute, East Zhangba Road 168, Xi'an 71065, Shaanxi Province, China) for his help in performing the X-ray experiment.

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